Exchangers of this type are employed, for example, in fast neutron nuclear reactors of the integrated type where the heat evolved by the reactor core is removed by virtue of the primary sodium which is contained in the reactor vessel and in which the reactor core is submerged. The heat is transmitted to the steam generator through the intermediary of secondary sodium which is heated by the primary sodium in heat exchangers immersed in the primary sodium filling the vessel, called intermediate heat exchangers. Secondary sodium enters these intermediate exchangers through their upper part, above the slab closing the reactor vessel, and also has to leave the intermediate heat exchanger through this upper part.
Such heat exchangers comprise a tube bundle having an annular shape and straight vertical tubes in which the secondary sodium is made to circulate upwards. The tube bundle comprises annular tube plates in its lower part and in its upper part, respectively. The secondary sodium is led under the lower tube plate, for circulation through the tubes of the bundle, by a central vertical duct passing through the entire height of the exchanger and joined in its upper part to a supply duct for secondary sodium. At the exit from the bundle, the secondary sodium enters an annular duct arranged coaxially relative to the secondary sodium entry duct and likewise opening into the upper part of the exchanger. The tube bundle is immersed in the primary sodium, which is circulated in contact with the outer surface of the bundle tubes by means of circulating pumps immersed in the primary sodium contained in the reactor vessel.
The secondary sodium entry duct under the lower plate of the tube bundle consists of a cylindrical double wall with a vertical axis also forming the inner wall of the tube bundle. This cylindrical double wall consists of a first shell welded to the two tube plates of the bundle, along their inner edges and extended upwards as far as the upper level of the exchanger, and of a second cylindrical shell coaxial with the first, fixed to the lower plate of the tube bundle along its inner edge and joined at its upper part to the secondary sodium entry duct. This second shell is arranged inside the first, the two shells being joined at their upper part by a leak-tight expansion joint making it possible to establish a gas atmosphere for thermal insulation (for example of argon) in the annular space between the two shells forming the inner wall of the tube bundle and the secondary sodium entry duct. This gas layer also allows the secondary sodium entering the heat exchanger to be insulated thermally from the hot secondary sodium leaving the tube bundle.
In large-scale heat exchangers and in particular in the intermediate exchangers in fast neutron nuclear reactors which operate at elevated temperatures, there are high temperature gradients which are represented by stresses of a high amplitude in the components forming these exchangers.
In particular, the outer shell of the secondary sodium entry duct, which is welded both to the lower tube plate and to the upper tube plate of the bundle, is stressed very severely. The whole unit formed by this shell, the tubes of the bundle and the tube plates is, effectively, a single unit and hyperstatic. In this assembly, the various components, for example the shell and the bundle tubes, have very different stiffnesses.
The heat exchanger is therefore subject to high differential expansions and deformations in its central part comprising the inner wall of the tube bundle.